1. Field of the Invention
The present invention relates in general to a synthetic cathode material for a molten salt battery and, more particularly, to a process of providing and using synthetic pyrite for use as a cathode in a thermal battery.
2. Description of Related Art
It is known to use a thermal battery containing a molten salt, a lithium or lithium alloy anode, and an iron disulfide (pyrite) cathode in applications requiring a high power output. These batteries, which have been successfully used in a number of military applications, include iron disulfide cathode material obtained as benefacted or from natural occurring pyrite deposits, or as a byproduct of flotation concentrate from the processing of base or noble metal ores.
It is highly unusual to find a natural occurring substance used directly as a principal ingredient in a chemical system requiring components with extremely uniform properties. In this case, the availability and quality of pyrite have caused persistent problems for thermal battery manufacturers. In recent times, it has been very difficult to obtain this material from domestic sources. Additionally, the quality of the pyrite has varied dramatically among sources and even between lots from the same source. It is axiomatic that the reproducibility of the cathode behavior would be improved by using pyrite of highly uniform physical and chemical properties.
It is known that the initial sulfidation of iron to pyrrhotite (FeS) is a very rapid and highly exothermic reaction. It is also known that the heat of this reaction may cause localized overheating causing deleterious sintering and even melting of residual iron. This is known to prevent sulfur from reacting further with the remaining iron powder. In addition, the localized overheating can raise the temperature above the decomposition temperature of pyrite, thus preventing its formation. In addition, sulfidation of iron in packed beds below sintering conditions using reduced sulfur vapor pressures has been found to be so slow as to make this method of little practical use.
It is, therefore, desirable to have a process for the production of pure synthetic pyrite with uniform physical and chemical properties which will provide a new raw material with minimum variations in processing and performance capability, thereby ensuring high quality thermal batteries.
It is also desirable to have available a synthetic pyrite which will provide a domestic source of feed that is not subject to periodic interruption by national events such as strikes and mine closings.
It is also desirable to have an improved cathode which will provide improved voltage regulation in thermal batteries made with synthetic pyrite cathode.
Further, it is desirable to provide a practical sulfidation process for producing pyrite which can be conducted below the sintering temperature and decomposition temperature of pyrite.